The present invention relates to a radar attenuating composition, and more specifically, to a paint material to be used on the surfaces of military targets for the purpose of protecting them from being detected, located and/or recognized by radar over a broad radiation spectrum.
Radiation over a broad spectrum may be used to detect, locate, or to recognize targets of a military importance. Microwave radiation over a wide spectrum from 5 to 18 GHz and at 35.+-.1 GHz has been used for long-range surveillance to detect, locate and recognize military targets at distances of from a few to hundreds of kilometers. This application is referred to as surveillance radar.
The reason for the absence of frequencies between 18 and 34 GHz is due to atmospheric absorption. Although most of the radiation is absorbed by the atmosphere in this range, a sufficient amount travels small distances that is, less than 1 kilometer, so that it can be used as seeker or guidance radar. At radar frequencies higher than about 36 GHz, the microwave radiation is absorbed at relatively short distances in the atmosphere except at specific frequencies of 94.+-.1 GHz, 140.+-.1 GHz, and 220.+-.1 GHz. Seeker or guidance radar systems at these frequencies are generally missile borne to guide missiles to their targets. Seeker systems at other frequencies above 18 GHz are also used in conjunction with longer range surveillance or acquisition radar systems to guide missiles to selected targets.
Radiation originating from natural sources, such as the sun, may be reflected from a target or absorbed by the target and readmitted to be detected by a sensor sensitive to the reflected radiation of a known wavelength. Artificially produced electromagnetic radiation may be also generated and scanned across a field to be reflected by targets of interest. The more important types of radiation include acoustic, ultraviolet, visual, near infrared, thermal infrared, millimeter wave, radar, and laser. Of the above, ultraviolet, visual, near infrared, and laser radiation are typically reflected by the target to be sensed by a suitable sensor. By contrast, thermal infrared radiation is emitted from a target which has a surface temperature greater than that of its surroundings. Such targets typically take the form of a motor vehicle having an internal combustion engine. In addition, thermal infrared radiation may be generated by solar surface heating, and by friction as would result from the movement of tank treads or from the use of electric power. Sensors that are adapted to sense thermal infrared radiation also are capable of sensing extra-terrestrial or space radiation that is reflected by the target, typically at night. Another band of radiation of interest is millimeter wave radiation typically generated and directed towards targets. The millimeter wave radiation is reflected back from the target and is received by missile borne sensors to provide a reflected radiation signal for homing the missile toward the target. Similarly, radar radiation is generated by an airborne radar system to detect targets, the reflected radar radiation from the target being used to detect the presence or location and to identify the target.
Until very recently, radar frequencies greater than about 30 GHz were not used for military applications. Various types of countermeasures have been developed and used to void radar frequencies generally below about 18 GHz such as the use of radar absorbing material that absorbs incident radar. The utilization of heretofore radar absorbing materials has generally required thick applications of the respective materials which makes for bulkiness and difficulty in transportation or deployment. Their use generally resulted in degradation of the effected systems mobility. In the category of the radar absorbing material, is a radar absorbing paint which relies upon the absorption of incident microwaves by the use of materials such as ferrites. However, the use of these materials has also been encumbered by the disadvantages of the weight and thickness requirements. In addition, materials that completely absorb radar do not simulate the natural background which absorbs and scatters. When used on a ground target and observed by an airborne sensor, the target will appear as a "black hole".
For example, U.S. Pat. No. 4,173,018, Dawson et al, discloses a mixture for the attenuation of electromagnetic wave frequency as an anti-radar means using finely divided diameter of from 0.5 to 20 microns in an insulating binder wherein the mixture can be applied by painting or spraying. The mixture is effective generally for attenuating electromagnetic wave energy in the frequency range of from 2 to 10 GHz by the application of approximately a 0.04 inch thick coating. The particles used comprise approximately 90% of the weight of the mixture. U.S. Pat. Nos. 2,918,671 and 2,954,552 to Halpern, disclose coating propeller blades for absorbing incident microwave radiation at pre-selected wave lengths comprising finely divided particles such as aluminum graphite, copper and the like, dispersed substantially insulated from each other in a neutral binder such as waxes, resins, rubbers, and the like and protective layers for absorbing incident radio microwave radiation comprising ferro-magnetic flake-like particles such as steel dispersed quasi-insulated in a binder. U.S. Pat. No. 3,185,986, McCaughna et al, disclose a mixture of ferromagnetic materials which can be utilized to shield objects from detection by radar by applying the mixture as a coating to a base member by painting or spraying. U.S. Pat. No. 3,662,387, Grimes, discloses a radio microwave radiation absorbent layer containing ferrite material. U.S. Pat. Nos. 2,996,709 and 2,418,479, to Pratt, relate to the use of flexible electromagnetic radiation absorbent materials including thin metallic flakes such as ferro-magnetic flakes in paint films. U.S. Pat. No. 4,034,375, Wallin, discloses a laminated camouflage material including electrically conductive fibers such as stainless steel or graphite lying in a plane essentially parallel to a first overcoated layer and a second undercoating layer comprising non-woven flexible fibers with electrically non-conductive polymeric material.
Although the above applications of radar absorbing material have been found useful, there are, as mentioned generally disadvantages in their uses. As discussed above, the use of heretofore known radar attenuating devices have been specifically applied in the complete absorption of incident microwaves and, in the process, required to be used in amounts at least equal to 20 to 25% and generally greater so as to represent the greatest concentration or weight of components present in the resulting composition. Typically, the use of the ferrite materials will exceed 50% of the binder or carrier with which it is combined. Thus, the resulting radar absorbing composition, such as radar absorbing paint, will be extremely heavy as a result of the concentration of additive and thickness at which the composition is applied. Furthermore, they have been known to be particularly effective at frequency ranges much lower than that necessary and primarily useful in the absorption of microwave radiation, to the exclusion of sometimes desirable combinations of absorption and reflection of the incident radiation for purposes of camouflage.
Other methods have been suggested to defeat the effect of radar, such as in the use of decoys and clutter. Decoys are designed to have the same radar cross-sections as the real target and generally their cost factors have been far too high for most applications. Clutter generally consists of smaller corner reflectors that have large radar cross-sections. They have been used for military purposes as countermeasure means against radar detection by being dropped from an aircraft to provide a reflector for the radar waves which create a false echo, that is, an echo which is not emitted from the target. However, this type of application has generally been easily overcome by the use of moving target radar indicators and in the case of the use of clutter in the form of chaff dispersed in the atmosphere, the attenuating effect is very transient due to the fact that the individual chaff elements or dipoles separate from each other by wind and/or other atmospheric effects thus providing only a very temporary effect. When they are separated by more than two radar wavelengths, the chaff attenuation of the radar is no longer effective.